The present invention generally relates to a magnetic material, and more particularly to a tunneling magneto resistance (TMR) material.
Magnetoelectronic, spin electronic or spintronic devices have drawn a great deal of attention in the field of magnetics. These devices, which include magnetic random access memory (MRAM), magnetic field sensors, read/write heads for disk drives, and other magnetic applications, use giant magneto resistance (GMR) and tunneling magneto resistance (TMR) effects predominantly caused by electron spin rather than electron charge.
One class of spintronic device is formed of a GMR material or a TMR material. The basic structure of these two materials includes two magnetic layers separated by a spacer layer. In a GMR material, the spacer layer is conductive, while in a TMR material, the spacer layer is insulating. FIG. 1 illustrates an enlarged cross-sectional view of a TMR material 20 according to the prior art.
The TMR material 20, which is also referred to as a Magnetic Tunnel Junction (MTJ), has a first magnetic layer 22 and second magnetic layer 24 separated by an insulating spacer layer 26, which is also referred to as a tunnel barrier layer. The first and second magnetic layers 22,24 can be single layers of magnetic materials such as nickel, iron, copper, cobalt or alloys thereof. The tunnel barrier layer 26 is typically aluminum oxide (Al2O3), but may include any number of insulators, such as aluminum nitride or oxides of nickel, iron, cobalt or alloys thereof.
Without intending to be bound by theory and with particular reference to the enlarged view 28 of the interface 31 between the tunnel barrier layer 26 and second magnetic layer 24, as adsorbate atoms 30 are deposited on the surface 32 of the tunnel barrier layer 26, several types of layer growth are possible, and even in the absence of mixing between the atoms 30 and the surface 32 at the interface 31, the layer growth of atoms 30 that forms on the surface 32 may not be a preferable thin film.
The growth mode of the atoms 30 on the surface 32 is determined by several factors including: the mobility of the atoms 30 on the surface 32, the surface energy of the surface 32, the surface energy of atoms 30, and the binding energy of the atoms 30 to the surface 32 at the interface 31. In a majority of physical vapor deposition processes, the atoms 30 have sufficient energy for significant mobility on the surface 32, moving numerous atomic spacings before coming to rest. In this medium to high mobility environment, the atoms 30 will naturally form a film morphology, which minimizes the total energy of the atoms 30 on the surface 32. Thus, when the surface energy of the atoms 30 is high compared to the energy of the surface 32, a configuration will be favored, which minimizes the surface area of the atoms 30 at the expense of exposing some area of the surface 32, resulting in the formation of three-dimensional islands 34 of atoms 30 on the surface 32 during the initial stages of film growth. Conversely, if the energy of the surface 32 is higher than the atoms 30, the growth of the atoms 30 in an atomic layer-by-layer fashion over the surface 32 is preferred since this quickly covers the surface 32 with atoms 33 that form a surface that has a lower energy.
Strong bonding of the atoms 30 to the surface 32 favors the growth of an atomic layer of atoms 30 by limiting the mobility of the atoms 30 and by decreasing the total system energy through maximization of the contact between the atoms 30 and the surface 32. During layer-by-layer growth, the atoms 30 nearly complete a first atomic layer of atoms 30 on the surface 32 before forming the second atomic layer of atoms 30 on atoms 30. Three-dimensional growth (i.e., island growth) occurs when the atoms 30 tend to grow additional atom 30 on atom 30 layers rather than completing the first atomic layer of atoms 30 on the surface 32.
A film is generally considered to be continuous when it has covered over about 80% of a surface. When the growth mode is a layer-by-layer growth, the film is more likely to become continuous much faster than for an island growth mode. For island growth, it may take the equivalent of ten or more atomic layers of deposition before the film becomes continuous or substantially continuous. Such films are generally considered to be discontinuous and are composed of disconnected islands before enough material is deposited to make islands large enough to connect and form a substantially continuous layer. Furthermore, once a continuous film is formed it will be rougher than a film that is grown in a layer-by-layer manner.
It is often desirable to form a smooth and substantially continuous film on a substrate that is less than about ten atomic layers (i.e., less than about 20 xc3x85 thick). Prior to the present invention, it was impossible to form a smooth and substantially continuous layer that was less than about 20 xc3x85 if the film of material forms by island growth or any growth mode that is similarly three-dimensional. Even though a film with substantial island growth may become continuous with ten atomic layers, it will be much rougher than a film grown in a layer-by-layer manner as some areas will be only one or two atomic layers thick while other areas will be well over 10 atomic layers thick. While this layer-by-layer formation provides proper operation of a TMR material, it is desirable to form a TMR material having a substantially smooth and continuous magnetic layer that is less than about 20 xc3x85 as significant benefits would be realized with such a TMR material structure.
For example, double MTJs would significantly benefit from a substantially smooth and continuous ultra-thin magnetic layer as resonant effects in a double MTJ would be tunable if a magnetic layer is available having a 1-3 atomic layer thickness. (See, Xiangdong Zhang, Bo-Zang Li, Gang Sun, and Fu-Cho Pu, Phys. Rev. B, vol. 56, p 5484 (1997), and S. Takahashi and S. Maekawa, Phys. Rev. Lett. vol. 80, p 1758 (1998) for theoretical predictions of resonant effects that give a higher MR, etc. which are hereby incorporated by reference). In addition, magnetic bi-layers (i.e., two magnetic materials forming the first or second magnetic layer) in a single or multiple tunnel junction would increase thermal endurance if composed of a substantially smooth and continuous ultra-thin diffusion tunnel barrier layer grown on another tunnel barrier layer and a soft magnetic layer combination such that switching characteristics would not be adversely affect during device operation. Furthermore, a specific crystallographic phase could be obtained with the selection of the two magnetic materials forming the magnetic bi-layer in order to obtain desired magnetic properties, including, but not limited to coercivity, anisotropy, and magneto resistive ratio considerations. As may be appreciated, there are many desirable applications and attributes for a TMR material having a substantially smooth and continuous uniform ultra-thin magnetic layer.
Accordingly, it is -desirable to have a TMR material that includes a substantially smooth and continuous uniform magnetic layer with a thickness that does not exceed about 20 xc3x85, preferably does not exceed 15 xc3x85, and more preferably does not exceed about 10 xc3x85.